WE NOW know that President Obama’s national security team overwhelmingly supported providing arms to the rebels in Syria. On Thursday, Defense Secretary Leon Panetta told a Senate committee that he and Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, backed a plan that would have vetted, trained and armed selected opposition groups, which have been pleading for such U.S. support for more than a year. According to the New York Times, the strategy was developed by former CIA director David H. Petraeus and supported by former secretary of state Hillary Rodham Clinton.

As we have frequently argued, the rationale for such action is compelling. Syria’s civil war, which has killed more than 60,000 people, grows steadily worse and more dangerous for the United States and its allies. An opposition that once was a peaceful pro-democracy movement has been all but overtaken by jihadist organizations, including an al-Qaeda affiliate, that receive ample funding and weapons supplies from abroad.

As Obama administration officials have frequently said, the longer Bashar al-Assad’s regime survives, the worse the outcome will be for Syria, its neighbors — all of which are U.S. allies — and the United States. Failure to back more moderate forces, which have been chronically short of effective weapons, will prolong the war and could leave the United States and its allies confronting a postwar Syria in which they have no friends and al-Qaeda and other extremists are ascendant.

So why was the Petraeus plan rejected? According to the Times, Mr. Petraeus and Ms. Clinton were rebuffed when they presented the plan to the White House. At the time, Mr. Obama was in the midst of an reelection campaign in which he frequently assured voters that “the tide of war is receding.” Hopes of reviving the plan after the election were thwarted when Mr. Petraeus resigned and Ms. Clinton was sidelined by illness.

Mr. Obama’s reasons for quashing the Syria plan were surely not purely political. But the president’s only public explanation for his resistance, in a recent interview with the New Republic, amounted to excuse-making. He wondered why he should concern himself with Syria and not the civil war in the Congo, as if the United States cannot intervene in any war unless it does in all; he asked whether providing weapons to rebels would “trigger even worse violence,” ignoring the testimony of his own aides that, under his present policy, the carnage “every day . . . it gets worse,” as new Secretary of State John F. Kerry put it.

Mr. Kerry and some other administration officials continue to talk up far-fetched hopes that the Syrian war will be ended by a negotiated settlement in which Mr. Assad voluntarily steps down. Even that unlikely ending would require the regime to conclude that it cannot defeat the rebels, and for moderate forces to rise among the fragmented opposition. As long as the United States and its allies refuse to directly supply those forces with money, training and more powerful weapons, that is very unlikely to happen.